In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, Device-to-Device (D2D) communication has recently been introduced as a radio technology that enables (wireless) devices to communicate directly with each other, that is without routing the data paths through a network infrastructure. Potential application scenarios include, among others, proximity-based services where devices detect their proximity and subsequently trigger different services (such as social applications triggered by user proximity, advertisements, local exchange of information, smart communication between vehicles, etc.). Other applications include public safety support, where devices provide at least local connectivity even in case of damage to the radio infrastructure. A wireless device capable of D2D operation (such as D2D discovery and D2D communications) will hereinafter be denoted a D2D capable wireless device. In general terms, D2D operation is introduced as a generic term which may comprise transmission and/or reception of any type of D2D signals (e.g. physical signals, physical channel etc.) by a D2D communication capable wireless device and/or by D2D discover capable wireless device. D2D operation therefore includes D2D transmission, D2D reception, D2D communication, etc.
D2D capable wireless devices may only transmit D2D signals, or channels, in the uplink part of the spectrum (i.e., the part of the frequency interval dedicated for transmission from wireless devices to network nodes). Further, D2D operation by a D2D capable wireless device may only be in a half-duplex mode (i.e. the D2D capable wireless device may, for a given time/frequency resource, either only transmit D2D signals/channels or only receive D2D signals/channels, but not in the same time/frequency resource transmit and receive D2D signals/channels). Some D2D capable wireless devices may act as relays and hence relay signals to other D2D capable wireless devices.
Some control information for D2D communications is transmitted by D2D capable wireless devices and other control information for D2D communications is transmitted by network nodes. For example, D2D resource grants for D2D communication is transmitted via cellular downlink control channels. The D2D transmissions may occur on resources which are configured by the network or selected autonomously by the D2D capable wireless devices.
In general terms, D2D communication implies transmitting, by a transmitter of a D2D capable wireless device, D2D data and D2D communication control information with scheduling assignments (SAs) to assist a receiver of a D2D capable wireless device of the D2D data. D2D data transmissions take place according to configured patterns and in principle may be transmitted rather frequently. SAs are transmitted periodically. D2D transmitters that are within the network coverage may request network node resources for their D2D communication transmissions and receive in response D2D resource grants for SA and D2D data. Furthermore, a network node may broadcast D2D resource pools for D2D communication.
In general terms, D2D discovery messages are transmitted in infrequent periodic subframes. Network nodes may broadcast D2D resource pools for D2D discovery, both for reception and transmission.
D2D communication may support two different modes of D2D operation, called mode 1 and mode 2, respectively.
In mode 1, the location of the resources for transmission of the scheduling assignment by the broadcasting D2D capable wireless device comes from the network node. The location of the resource(s) for transmission of the D2D data by the broadcasting D2D capable wireless device comes from the network node.
In mode 2 a resource pool for scheduling assignment is pre-configured and/or semi-statically allocated. The D2D capable wireless devices on its own selects the resource for scheduling assignment from the resource pool for scheduling assignment to transmit its scheduling assignment.
Primary cell (PCell) interruption of one subframe occurs when a D2D capable wireless device switches its reception from D2D-to-WAN operation or from WAN-to-D2D operation (where WAN is short for wide area network and generally refers to any type of cellular communication network). This is because the receiver chain in the D2D capable wireless device needs to be retuned every time the operation is switched from WAN reception to D2D reception and from D2D reception to WAN reception. This applies to both D2D discovery and D2D communication capable wireless devices. Uplink resources between cellular (i.e., WAN) uplink and D2D operation should be partitioned to avoid, or minimize, the risk of the switching taking place in certain subframes, particularly subframe 0 and/or subframe 5, of the PCell. These subframes contain essential information such as primary and secondary reference signals (PSS/SSS) that are necessary for the wireless devices to perform cell search, and carrying out cell measurements. These subframes also contain master information block (MIB) and system information block (SIB1) information which is necessary for system information (SI) reading procedures. In addition to interruption that takes places due to such switching, there may be additional interruption of one subframe due to Radio Resource Control (RRC) reconfiguration procedure. While the switching interruption takes place for single receiving wireless device (e.g. a D2D discovery capable wireless device), the RRC reconfiguration interruption takes place for all types of D2D wireless devices (e.g. D2D Discovery capable and D2D Communication capable wireless devices).
A D2D capable wireless device is also interchangeably called a proximity-service (ProSe) capable wireless devices. Thus, a D2D discovery capable wireless device is also referred to as a wireless device capable of ProSe direct discovery and a D2D direct communication capable wireless device is also referred to as a wireless devices capable of ProSe direct communication. The link and/carrier that is used for the ProSe direct communication and ProSe direct discovery between wireless devices is referred to as a sidelink. The carrier used for the sidelink may be called a ProSe carrier or a D2D carrier. The sidelink can be on the same carrier (e.g. UL carrier in frequency division duplex (FDD)) as used for WAN operation (i.e., a cellular communication carrier) or it can be on a carrier (i.e., a ProSe dedicated carrier) which is not used for WAN operation.
A D2D capable wireless device can be configured for D2D operation on a PCell and/or on one or more secondary cells (SCells). The PCell and the SCells are primarily used for cellular operations, e.g. reception and/or transmission of cellular signals (i.e., WAN operation). Several D2D capable wireless devices may have the same PCell. If the D2D capable wireless devices are mainly configured on a PCell then significant amount of the resources (e.g. subframes) on the sidelink which shares the same carrier as of the PCell will be used, or at least reserved, for D2D operation. This may degrade the performance of the WAN operation, especially if the PCell carries control information associated with carrier aggregation (CA) operation. To maintain acceptable WAN performance the network node may assign limited amount of D2D resources on the sidelink which operates on the same carrier as of the serving cell of the D2D capable wireless device, e.g. the PCell. However insufficient resources allocated for D2D on the sidelink may degrade D2D performance, e.g. due to infrequent transmissions of vital signals/channels such as D2D synchronization, scheduling assignment, etc.
On the other hand, if the D2D operation is arbitrarily distributed on sidelinks on carriers of the PCell and SCells for different D2D capable wireless devices then some D2D capable wireless devices may not be able to communicate with each other. For example, a D2D capable wireless device on a sidelink with a first carrier (say, f1) may not be able to receive D2D signal transmitted using D2D operation from a sidelink on a second carrier (say, f2).
Hence, there is still a need for an improved balancing of D2D traffic in a communications network.